This application claims the priority benefit of U.K. Patent Application 0400632.6, filed Jan. 13, 2004, which is hereby incorporated by reference in its entirety.
This invention relates to the testing of mechanical properties of materials, and in particular to the testing of mechanical properties of materials for use in pavement construction. By “pavement” in this context is meant particularly surfaces intended to provide a carriageway or hard standing for vehicles, eg roadways, car parks, airport runways and taxiways, and the like.
Roads and other forms of pavement are commonly constructed using quarried aggregate that is compacted and optionally bound using a suitable binder, such as cement or bitumen. It is clearly desirable to be able to test the mechanical properties of such materials in order to ensure that the material being tested is suitable for its intended application. It may also be desirable to test samples of material taken from an existing pavement in order to monitor the performance of the pavement.
One established method for testing the mechanical properties of samples of bound aggregate, such as asphalt, uses apparatus known as the Nottingham Asphalt Tester, which is described, for example, by S F Brown in Proc. Instn Civ. Engrs Transp., (1995), 111, November, pp 289–297. However, apparatus such as the Nottingham Asphalt Tester may not be suitable for testing the mechanical properties of unbound or lightly-bound aggregates.
The mechanical properties of unbound and lightly-bound road construction materials are presently investigated either in situ using dynamic plate testing, which requires a full-scale pavement section to be constructed, or in a laboratory using a repeated load triaxial test, which is a relatively complicated and expensive procedure that is presently only practicable in universities and research establishments.
There is therefore a need for material testing apparatus that is simple and inexpensive to operate but generates a useful set of data regarding the relevant mechanical properties of unbound or lightly-bound aggregate for use in road construction.
There has now been devised an improved apparatus and an improved method for the testing of mechanical properties of a material, which overcome or substantially mitigate the above-mentioned and/or other disadvantages associated with the prior art.
According to a first aspect of the invention, there is provided apparatus for testing the mechanical properties of a material, which apparatus comprises a sample container adapted to be filled with a sample of the material, and a housing within which the sample container is, in use, fixedly mounted, the sample container having a first, open face via which a compressive load can be applied to the sample and a second face disposed orthogonally to the first, open face, at least a portion of said second face being resiliently displaceable, outwardly of the sample container, in response to deformation of the sample brought about by application of the load.
According to a further aspect of the invention, there is provided a sample container for use in testing the mechanical properties of a material, the sample container being adapted to be filled with a sample of the material, and having a first, open face via which a compressive load can be applied to the sample and a second face disposed orthogonally to the first, open face, at least a portion of said second face being displaceable, outwardly of the sample container, in response to deformation of the sample.
According to a further aspect of the invention, there is provided a method of testing the mechanical properties of a material, which method comprises the following steps:    (a) providing apparatus as described above;    (b) introducing a sample of the material into the sample container;    (c) engaging the sample container with the housing;    (d) applying a compressive force to the sample; and    (e) measuring movement of the resiliently-displaceable second face, or resiliently-displaceable portion thereof, in response to said compressive force.
The apparatus and method according to the invention are advantageous principally because the mechanical properties of many different materials, including unbound and lightly-bound aggregate, can be accurately investigated in a simple and cost-effective manner. In particular, the apparatus and method according to the invention provide more accurate data than conventional methods such as in situ dynamic plate testing, static plate testing or California Bearing Ratio (CBR) testing, and are simpler and more cost-effective than repeated load tri-axial testing. In addition, samples of material can be prepared and stored in the container for a period of time before being engaged with the housing for testing.
Using the apparatus according to the invention, mechanical property data (eg elastic stiffness and permanent strain) may be obtained under repeated loading for unbound and lightly-bound material. This may not be possible using conventional methods as such material may collapse under loading if not supported. The fact that using the method and apparatus of the invention the material under test is only partially confined means that the measured behaviour more accurately reflects that to be expected on site.
The second face, or part of the second face, that is resiliently displaceable in use, is preferably a panel that is releasably fixable relative to the remainder of the sample container. The container preferably comprises a pair of opposed releasably fixable panels that form part of the wall of the container. Most preferably, the container has the form of a square or rectangular box having an open upper face, a square or rectangular base, one pair of opposed side walls that are fixed to the base, and a pair of opposed rectangular panels that constitute the other side walls of the container and are releasably secured to the rest of the container.
Thus, the container preferably has the form of an open-topped box. At least one, and more preferably an opposed pair, of the side walls of the box are, in use, displaceable outwardly in response to the compressive force applied via the open top of the container.
Most preferably, the releasable panels that make up one, or more preferably two, of the sides of the container can be clamped to the rest of the container, so that the panels are held in place prior to a measurement being made.
Once the container has been engaged with the housing, the panels are released from the container so as to be displaceable in response to deformation of the sample of material. Most preferably, the housing includes resilient means that engage each panel, displacement of each panel taking place against the action of the resilient means.
The container is preferably provided with handles, and is preferably less than about 30 kg in weight when charged with a sample of material, thereby facilitating manual handling of the container during use. Preferably, the container is constructed from stainless steel plate having a thickness of between 4 and 8 mm, for example about 6 mm. The internal surfaces of the container are preferably lined with a layer of material having a lower coefficient of friction than steel, such as polytetrafluoroethylene (PTFE), thereby reducing the coefficient of friction between the container and the sample of material. The layer of low-friction material is preferably less than 1 mm in thickness, and most preferably less than 0.6 mm in thickness.
The sample of material may be any material that can be packed with a close fit into the container. In the field of road and pavement construction, however, the material is most likely to comprise soil or aggregate with or without a suitable binder, such as lime, cement or bitumen. If the material includes a binder, the material is preferably introduced into the container while the binder is uncured. The sample of material is most preferably compacted within the container, and then any binder may be allowed to cure, so as to give the sample of material a form similar to that of the material in situ in a road or the like. In addition, water may be added to the sample of material before testing so as to simulate drainage or saturation conditions.
Preferably, means are provided for compacting the sample of material before the container is engaged with the housing. Such means preferably takes the form of a conventional vibrating hammer.
Most preferably, a compaction jacket is provided for supporting the walls of the container while the sample of material is compacted. The compaction jacket is preferably removed after compaction and prior to the container being engaged with the housing.
The compaction jacket is preferably adapted to fit closely about the walls of the container. Where the container has the form of a box having an open upper face, the compaction jacket preferably has opposed walls of variable separation. In particular, opposed walls of the compaction jacket are preferably mounted for at least a limited range of movement relative to the other walls, and the compaction jacket preferably includes means, eg threaded rods and tightening nuts, for drawing the walls into close abutment with the container. In use, therefore, the container is introduced into the compaction jacket, or the compaction jacket is fitted about the container, and the walls of the compaction jacket are urged into abutment with the sample container to provide support thereto during compaction.
The functionality of the sample container, such that the sides can be fixed for compaction, but the second face released for testing, greatly enhances the possible throughput of samples that can be tested compared with conventional techniques, and reduces costs.
The housing preferably includes means for fixing the container within the housing. Most preferably, the housing is provided with means for clamping the container within the housing, for example clamping bolts that are threadably engaged with a wall of the housing. In addition, the housing is preferably fixed relative to the means by which a compressive force is applied to the sample during use.
The housing preferably includes resilient means that engage each panel of the container, during use, so that each panel moves resiliently relative to the housing. The resilient means preferably comprises an abutment plate that is resiliently mounted relative to a wall of the housing, and that engages a panel of the container during use. Preferably, the resilient means comprises one or more resilient members, such as compression springs, that act between the abutment plate and either a wall of the housing or a component that is fixed, during use, relative to a wall of the housing.
In a preferred embodiment, the one or more resilient members act between the abutment plate and an intermediate plate that is fixed, during use, relative to a wall of the housing. In this case, the intermediate plate is preferably mounted relative to a wall of the housing such that the separation of the intermediate plate from the wall of the housing is variable. The variable separation is preferably achieved by means of the intermediate plate being slidably mounted relative to a wall of the housing, and a separation member that is threadably engaged to either a wall of the housing or a component that is fixed relative to the housing determining the separation of the intermediate plate from the wall of the housing. In this way, the separation of the intermediate plate from the wall of the housing may be increased until the abutment plate engages with a panel of the container. Most preferably, the stiffness of the one or more resilient members is adjustable.
Preferably, the housing has the form of a rectangular box with an open upper face through which the container is received. Preferably, the housing is constructed from stainless steel plate having a thickness of between 4 and 8 mm, for example about 6 mm. The walls of the housing may be strengthened with strengthening components that are fixed to the outer surface of the walls of the housing. For example, longitudinally orientated strengthening ribs, eg square-section steel tubing, may be fixed to the walls of the housing, and strengthening rings may be fixed about any openings in the walls of the housing. In order to facilitate the removal of any liquids that would otherwise accumulate within the housing, one or more drainage holes may be provided in the base of the housing.
The means for applying a compressive force to the sample of material may be generally conventional. Such means preferably comprises a loading plate for contacting the sample of material, a loading ram for applying force to the loading plate, and a loading frame to which the housing is fixed. Most preferably, the means for applying a compressive force to the sample of material takes the form of similar means used in conventional apparatus such as a Nottingham Asphalt Tester. The compression means is preferably computer controlled using suitable software, and is preferably capable of applying loads up to around 10 kN. Most preferably, the compression means is capable of applying repeated loads, in particular repeated loads of a predetermined magnitude, duration and frequency of repetition. The loading plate is preferably connected to the loading ram by a half-ball connector to ensure an even contact between the loading plate and the sample of material.
Deformation of the sample of material is preferably measured by measuring the displacement of each resiliently-displaceable panel of the sample container and/or the displacement of a loading plate of the compression means during testing. Most preferably, both the displacement of each panel and the displacement of the loading plate of the compression means is measured. Each displacement is most preferably measured using at least one linear displacement transducer, which is most preferably a Linear Variable Differential Transformer (LVDT). Each linear displacement transducer preferably communicates with a microcomputer which conducts analysis of the data received. In particular, the microcomputer preferably calculates, from the data received from the linear displacement transducers, any one of the elastic stiffness, the permanent shear strain, and the permanent volumetric strain, of the sample of material.
For each test, a range of different stress levels are preferably applied to the sample of material. Most preferably, each stress level corresponds to a different layer of the road or pavement for which the material is intended. For example, the stress levels applied to the sample of material might range from 50 to 200 kPa, and three or more different stress levels may be applied to each sample of material.
The invention will now be described in greater detail, by way of illustration only, with reference to the accompanying drawings, in which